Grease composition for constant velocity joint

ABSTRACT

A grease composition for a constant velocity joint is provided, which comprises a mineral oil as a base oil along with an alicyclic-aliphatic diurea thickener, a sulfur-ester-based additive, molybdenum dithiocarbamate (Mo-DTC), molybdenum dithiophosphate (Mo-DTP) and zinc dithiophosphate (Zn-DTP) as additives. The composition improves NVH performance by lowering a generated axial force and P.R. (plunging resistance) while increasing abrasion resistance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2008-0068744 filed Jul. 15, 2008, the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to a grease composition for a constant velocity joint.

(b) Background Art

A constant velocity joint receives a driving or braking torque and transmits the torque regardless of angle of deviation in two axes connected to the joint. A grease applied to the constant velocity joint reduces a lateral direction shaking, which is generated in sudden acceleration or braking in low gear and causes shudder vibration of a vehicle. The grease also prevents shudder vibration of a vehicle with an automatic transmission in idle mode. However, when the grease has a low abrasion resistance, an abnormal abrasion can occur between a wheel and a roller depending on angle of deviation of a joint, thereby causing the shudder vibration.

SUMMARY OF THE DISCLOSURE

One of the objects of the invention is to provide a grease composition for a constant velocity joint that can decrease friction between metals while increasing abrasion resistance and improving lubricating properties.

The grease composition according to one aspect of the present invention comprises a mineral oil as a base oil along with a urea-based thickener, a sulfur-ester-based additive, molybdenum dithiocarbamate (Mo-DTC), molybdenum dithiophosphate (Mo-DTP) and zinc dithiophosphate (Zn-DTP).

The above and other aspects of the invention is discussed infra.

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiments of the present invention. The embodiments are described below so as to explain the present invention by referring to the figures.

A grease composition for a constant velocity joint according to the present invention comprises: (a) 75.0-88 wt % of a base oil; (b) 7.0-10.0 wt % of a thickener comprising an aliphatic diurea and an alicyclic diurea in a mixing ratio of 1:2.2-1:2.4 by weight; and (c) 3.6-15 wt % of an additive comprising a sulfur-ester-based additive, molybdenum dithiophosphate (Mo-DTP), molybdenum dithiocarbamate (Mo-DTC) and zinc dithiophosphate (Zn-DTP).

As the base oil, although any known base oils for a grease composition for a constant velocity joint can be used in the present invention, a mineral oil is preferred. Examples of the mineral oil include solvent-purified or hydrogenated paraffin-based mineral oils and naphthene-based mineral oils.

Preferably, the base oil may have a dynamic viscosity of 60-100 mm²/s at 40° C. If the dynamic viscosity is lower than 60 mm²/s, the base oil may be evaporated due to insufficient thermal resistance. If the dynamic viscosity is higher than 100 mm²/s, it will increase torque and heat emission.

Suitably, the base oil is contained in a grease composition in the amount of 75.0-88.0 wt %. If the amount of the base oil is less than 75.0 wt %, the grease will not be applicable due to excessive solidification. If the amount is more than 88.0 wt %, it will cause the oil separation too high and also cause softening problem.

The thickener used in the present invention may comprise (i) an alicyclic diurea of Formula 1 prepared by reacting diphenylmethane-4,4-diisocyanate (MDI) with cyclohexyl amine and (ii) an aliphatic diurea of Formula 2 prepared by reacting toluene diisocyanate (TDI) and oleyl amine in an appropriate mixing ratio. Suitably, the thickener is contained in a grease composition preferably in the amount of 7.0-10.0 wt %. When the amount is less than 7.0 wt %, the composition may result in a liquid phase with high penetration, thus easily causing oil separation. When the amount is more than 10.0 wt %, fluidity can be lowered due to solidification of grease.

R¹NH—CO—NH—C₆H₄-p-CH₂—C₆H₄-p-HN—CO—NHR¹  [Formula 1]

where R¹ is a C₆ cyclohexyl group.

2,4-[R²NH—CO—NH]₂—C₆H₃—CH₃  [Formula 2]

where R² is a C₁₈ oleyl group.

A urea-based thickener can be prepared by reacting isocyante with an amine compound. To avoid unreacted amine group, isocyante is preferred to be mixed with an approximately equivalent amount of the amine compound.

Moreover, a preferable mixing ratio of the aliphatic diurea and the alicyclic diurea is 1:2.2-1:2.4 considering the hardening property at high temperature and the softening property under sheared conditions.

The additive is contained in a grease composition preferably in the amount of 3.6-15 wt %, and the additive may preferably include a sulfur-ester-based additive, Mo-DTP, Mo-DTC and Zn-DTP.

In particular, Mo-DTC and Mo-DTP, which are used for improving friction and avoiding abrasion, have a structure of the following Formulas 3 and 4, respectively.

[R³R⁴N—CS—S]₂—Mo₂O_(m)S_(n)  [Formula 3]

where R³ and R⁴ are independently a C₁-C₂₄ alkyl group; and m is an integer of 0-3 and n is an integer of 1-4 with m+n being 4.

where R⁵, R⁶, R⁷ and R⁸ are independently a C₁-C₂₄ primary or secondary alkyl or a C₆-C₃ aryl group.

The additive is used for improving friction property, extreme pressure property and abrasion resistance. A preferable amount of Mo-DTC is 1.5-4.0 wt % relative to the total weight of the grease composition. When the amount is more than 4.0 wt %, no improvement of properties can be anticipated. When the amount is less than 1.5 wt %, the improvement of friction property can be insufficient. A preferable amount of Mo-DTP is 0.1-1.0 wt % relative to the total weight of the grease composition. If the amount is more than 1.0 wt %, the increase in abrasion resistance can level off. If the amount is less than 0.1 wt %, the abrasion resistance can be lowered.

The sulfur-ester-based additive is used in the amount of 1.0-3.0 wt % relative to the total weight of the grease composition for maintaining a stable friction coefficient and abrasion resistance relative. If the amount is outside the aforementioned range, friction and abrasion properties become unstable.

Zn-DTP is used for improving the friction property of molybdenum dithiocarbamate in the amount of 1.0-3.0 wt % relative to the total weight of the grease composition. If the amount is more than 3.0 wt %, the increase in abrasion resistance can level off. If the amount is less than 1.0 wt %, the abrasion resistance can be insufficient.

In an embodiment, the grease composition may further comprise the conventional additives such as extreme pressure additives, antioxidants and anticorrosive agents besides the aforementioned additives, i.e., a sulfur-ester-based additive, Mo-DTP, Mo-DTC and Zn-DTP.

Any known extreme pressure agent can be used in the present invention to improve the load resistance and the extreme pressure property. Examples of the extreme pressure agent include organic zinc compounds such as zinc dithiocarbamate, zinc dithiophosphate and zinc phenate; organic antimony compounds such as antimony dithiocarbamate and antimony dithiophosphate; organic bismuth compounds such bismuth naphthenate and bismuth dithiocarbamate; organic sulfonate, phenate or phosphenate of alkali metal or alkaline earth metals; and organic metal compounds such as of gold, silver, titanium or cadmium. Also, sulfur-based extreme pressure agents can be used. Examples of sulfur-based extreme pressure agent include sulfides such as dibenzyldisulfide, polysulfide, oil sulfides, ashless carbamate compounds, thiourea-based compound and thiocarbonates. In addition, phosphate-based extreme pressure agents can be used. Examples of phosphate-based extreme pressure agents include phosphate esters such as trioctylphosphate, tricredylphosphate; and acid phosphate esters such as phosphorous ester and acid phosphorous ester. Furthermore, chlorinated extreme pressure agents such as chlorinated paraffin and solid lubricants such as molybdenum disulfide, tungsten disulfide, graphite, polytetrafluoroethylene (PTFE), antimony sulfide, boron nitride can be used.

Examples of the antioxidants include age resistors and ozone-deterioration-preventing agents, which are conventionally contained in rubber, plastics and lubricants. Examples of such compounds include amine compounds such as phenyl-1-naphthyl amine, phenyl-2-naphthyl amine, diphenyl-p-phenylene diamine, dipyridyl amine, phenothiazine, N-methylphenothiazine, N-ethylphenothiazine, 3,7-dioctylphenothiazine, p,p′-dioctyldiphenyl amine, N,N′-diisopropyl-p-phenylene diamine and N,N′-di-sec-butyl-p-phenylene diamine; and phenol-based compounds such as 2,6-di-tert-dibutylphenol.

Examples of the anticorrosive agents include sulfonates of alkali metal or alkaline earth metal such as ammonium salt, barium, zinc, calcium and magnesium of organic sulfonate; organic carbonates; organic phenates; organic phosphonates; alkyl or alkenyl succinic acid derivatives such as alkyl or alkenyl succinic acid ester; partial esters of polyhydric alcohol such as sorbitan monostearate; hydroxyl fatty acids such as oleoyl sarcosine; mercaptofatty acids or metal salt thereof such as 1-mercaptosteatic acid; higher fatty acids such as stearic acid; higher alcohols such as isostearyl alcohol; ester of higher alcohol and higher fatty acid; thiazoles such as 2,5-dimercapto-1,3,4-thiadiazole and 2-mercaptothiadiazole; imidazoles such as 2-(decyldithio)-benzoimidazole and benzoimidazole; disulfides such as 2,5-bis(dodecyldithio)benzoimidazole; phosphate esters such as trisnonylphenylphosphite; thiocarbonate ester such as dilaurylthiopropionate; and nitrites.

The urea-based thickener improves the mechanical stability and water resistance of the grease composition. The sulfur-ester-based additive, Mo-DTP, Mo-DTC and Zn-DTP added to a mixture of the aforementioned base oil and urea thickener improves friction property and abrasion resistance of the grease composition, thereby maintaining Lubricating effect for a long time.

EXAMPLES

The following examples illustrate the invention and are not intended to limit the same.

Preparation Example 1 Preparation of Alicyclic Diurea (Thickener)

4,500 g of base oil and 558 g of diphenylmethane-4,4′-diisocyanate were mixed and heated at 70-80° C. A mixture of 4,500 g of base oil and 442 g of cyclohexyl amine was added to the heated mixture. The resulting mixture was sufficiently mixed, heated up to 170° C. and placed at room temperature, thus providing a base urea grease.

Preparation Example 2 Preparation of Aliphatic Diurea (Thickener)

4,500 g of base oil and 362.7 g of toluene-2,6-diisocyanate were sufficiently mixed, and added with a mixture of 4,550 g of base oil and 537.3 g of oleyl amine. The resulting mixture was sufficiently mixed, heated up to 170° C. and placed at room temperature, thus providing a base urea grease.

Examples 1-3 and Comparative Examples 1-2

Grease compositions for a constant velocity joint were prepared by using ingredients as shown in Table 1. A mineral oil (dynamic viscosity at 40° C.: 95 cSt) was used as a base oil. A urea-based thickener was prepared by mixing the aliphatic diurea (Preparation Example 2) and the alicyclic diurea (Preparation Example 1) in the mixing ratio of 1:2.3.

TABLE 1 Ex. Comp. Ex. Ingredients (wt %) 1 2 3 1 3 Base oil Mineral oil 85.7 85.5  87.3  86   87.5  Thickener Alicyclic diurea 6.6 6.3 6.3 9.5 — Aliphatic diurea 2.9 2.7 2.7 — 9.0 Additive Molybdenum 2.2 2.0 1.7 — 2.0 dithiocarbamate¹⁾ Molybdenum 0.1 — — 0.1 — dithiophosphate²⁾ Sulfur ester³⁾ 1.0 2.0 1.0 1.0 — Zinc dithiophosphate⁴⁾ 1.5 1.5 1.0 1.5 1.5 ¹⁾Molybdenum dithiocarbamate: Sakuralube 200 of Adeka Co., Ltd. ²⁾Molybdenum dithiophosphate: Sakuralube 300 of Adeka Adeka Co., Ltd. ³⁾Sulfur ester: Lubrizol 5333 of Lubrizol Co., Ltd. ⁴⁾Zinc dithiophosphate: RC3180 of RheinChemie Co., Ltd.

Test Examples Measurement of Properties

Properties of the grease compositions for a constant velocity joint prepared in Examples 1-3 and Comparative Examples 1-2 were measured as follows, and the results are presented in Table 2.

1) Worked penetration was measured according to ASTM D 217,

2) propping point was measured according to ASTM D 566.

3) Oil separation was measured according to ASTM D6184.

4) Friction coefficient (SRV) was measured under the following conditions according to ASTM D 5707-05 (A standard method for measuring the friction property and the abrasion resistance of a grease by using high-frequency SRV tester)

Load: 100N, 300N and 500N

Speed: 10 Hz

Distance: 4 mm

TABLE 2 Ex. Comp. Ex. Properties 1 2 3 1 3 Worked penetration 321 325 323 325 325 (at 25° C.) Dropping point (° C.) 235 237 234 235 232 Oil separation (wt %) 0.7 0.6 0.7 0.7 0.7 Friction coefficient 0.060 0.070 0.068 0.09 0.105 (SRV)

As shown in Table 2, the grease compositions prepared by using a urea-based thickener with a relatively high limit heat-resistant temperature in Examples 1-3 were ascertained as superior in oil separation and dropping point showing durability and operability at high temperature. Furthermore, the grease compositions prepared in Examples 1-3 were superior to those of Comparative Examples in friction property because a sulfur-ester-based additive, Mo-DTP, Mo-DTC and Zn-DTP were used.

A grease composition for constant velocity joint of the present invention is effective in stably maintaining friction coefficient by using Zn-DTP and a sulfur-ester-based additive. A grease composition for constant velocity joint of the present invention is also superior in lowering friction coefficient between metals, increasing abrasion resistance and decreasing a generated axial force and P.R. value by using Mo-DTP and Mo-DTC.

The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents. 

1. A grease composition for constant velocity joint comprising: (a) 75.0-88 wt % of a base oil; (b) 7.0-10.0 wt % of a thickener comprising an aliphatic diurea and an alicyclic diurea in a mixing ratio of 1:2.2-1:2.4 by weight; and (c) 3.6-15 wt % of an additive comprising a sulfur-ester-based additive, molybdenum dithiophosphate (Mo-DTP), molybdenum dithiocarbamate (Mo-DTC) and zinc dithiophosphate (Zn-DTP).
 2. The composition of claim 1, wherein the base oil is a mineral oil having a dynamic viscosity of 60-100 mm²/s at 40° C.
 3. The composition of claim 1, wherein the alicyclic diurea is a compound of the following Formula 1: R¹NH—CO—NH—C₆H₄-p-CH₂—C₆H₄-p-HN—CO—NHR¹  [Formula 1] wherein R¹ is a C₆ cyclohexyl group.
 4. The composition of claim 1, wherein the aliphatic diurea is a compound of the following Formula 2: 2,4-[R²NH—CO—NH]₂—C₆H₃—CH₃  [Formula 2] wherein R² is a C₁₈ oleyl group.
 5. The composition of claim 1, wherein the additive comprises 1.0-3.0 wt % of a sulfur-ester-based additive, 0.1-1.0 wt % of molybdenum dithiophosphate, 1.50-4.0 wt % of molybdenum dithiocarbamate, and 1.0-3.0 wt % of zinc dithiophosphate.
 6. The composition of claim 1, wherein the molybdenum dithiocarbamate is a compound of the following Formula 3: [R³R⁴N—CS—S]₂—Mo₂O_(m)S_(n)  [Formula 3] wherein R³ and R⁴ are independently a C₁-C₂₄ alkyl group; and m is an integer of 0-3 and n is an integer of 1-4 with m+n being
 4. 7. The composition of claim 1, wherein the molybdenum dithiophosphate is a compound of the following Formula 4:

wherein R⁵, R⁶, R⁷ and R⁸ are independently a C₁-C₂₄ primary or secondary alkyl or a C₆-C₃₀ aryl group. 